Composition comprising an acyl peroxide cured polyester and an alkaline carbonate



Patented Sept, 7,1948

UNITED srArEs PATENT oF-rice I oomrosrrron .oourmsmann Acrr. ran oxmr:cumin roLrns'rnn AND ANY amu- LINE CARBONATE Barnard S. llluasnmmlt, N.1., assignor to Bell Telephone Laboratories Incorporated, New

Yorh'N. Y., a corporation or New York 7 Serial No. 485,182

This invention relates to cured polyester .synthetic rubber compounds orrigid crystalline compounds which contain pigments or other substanceswhich increase the life of these compounds and in some cases impart tothose compounds which are rubber-like a tensile strength which isoutstanding as compared tc the tensile strengths imparted by otherpigments.

A new class of synthetic rubbers has recently been developed which ismanufactured by curing plastic or thermoplastic polyesters of highmolecular weight with benzoyl peroxide or other substances having asimilar action. The curing is accomplished by intimately mixing theuncured polyester with the curing agent and heating the and elevatedtemperatures.

According to the present invention, the life of the cured polyestersdescribed above is increased by intimately incorporating in thepolyester finely divided, substantially neutral solid substances havingan alkaline buffering action, particularly alkaline carbonates. Of thealkaline carbonates, those which are insoluble in water, which areobtainable in finely divided form and which are good reinforcingpigments'for the rubber, as well as stabilizing agents, are the mostdesirable.

Thus, although sodium bicarbonate and the alkaline earth carbonates,including magnesium carbonate, are suitable for increasing the life ofthe cured polyesters, the best results are obtained, particularly withrubber-like polymers, by using the finely divided calcium carbonates.The most desirable bufiering pigment is an ultra-fine precipitatedcalcium carbonate having a particle size of the order of .03 micron orless. A pigment of this type is sold commercially under the tradenameoffKalvan.

Not only does this ultra-fine calcium carbonate provide aeured product.which is more stable, but it also imparts to the cured rubbers a tensilestrength which is approached by no other reinforcing pigment yet tested,except one, which does not have an undesirable effect on the lite oi thepolyester. The only other pigment yet found which has an equivalentreinforcing power and no deleterious efiect on the stability of thepolymer is an extremely pure red oxide'of iron sold under the name of"Mapioo 297."

The remarkable reinforcing action of this ultrailne calcium carbonate isillustrated by the fact that a polyester made up of 50 mol per cent.ethylene glycol, 50 mol per cent isopropylenejglycol, 9'7 mol per centsuccinic acid and 3 mol per cent maleic acid, which when cured with 1.5per cent benaoyl peroxide in the absence of any pigment had a tensilestrength not substantially greater than 100 pounds per square inch, hada tensile strength of 2500 pounds per square inch when compounded with75 per cent by weight of Kalvan prior to curing. The same polyester,when compounded with an equivalent amount of 20 Atomite (a coarsercalcium carbonate having a particle size of between 1 and 2 microns) andcured, had a tensile strength of only about 1200 pounds per square inch.

The peroxide cured polyester rubbers to which the present invention isapplicable are described and claimed in the copending application of C.S. Fuller, Serial No. 485,202, filed on the same day as the presentapplication. The uncured polyester gums, from which the cured rubbersare derived, may be strictly linear polyesters of high molecular weightcontaining no non-benzenoid carbonto-carbon unsaturation, which areprepared by the superesteriflcation of a glycol with a dicarboxylicacid, or of a hydroxy acid with itself, in a manner similar, forinstance, to that described for crystalline polyesters in U. S. Patents2,071,-

250 and 2,249,950.

', Polyesters which are essentially similar but which contain limitedamounts of olefinic or nonbenzenoid unsaturation may also be cured toform synthetic rubbers to which the present invention is applicable.These partially unsaturated polyesters may be. prepared in the samemanner as the fully saturated polyesters except that one or more of the,ingredients of the reaction mixture from which they are preparedcontains a properly limited amount of unsaturated carbon-tocarbon bond.

The polyester gums, used for forming cured substances which haverubber-like properties at room temperatures, arev extremely viscousliquids, which at room temperatures have a consistency somewhat similarto that of milled crepe rubber, or else they are somewhat flexiblerubbery solids 55 oi slight crystallinity which melt readily at tem- 3peratures up to about 20 C. above room temperature to form viscousliquids of a consistency similar to the normally liquid polyesters.These latter substances possess suiiflcient flexibility, being largelyamorphous, to be milled directly on cold rolls, where they are almostinstantly reduced to a viscous liquid state by the temperature riseinduced by milling.

The more highly crystalline polyesters, when cured, form hard, rigidsubstances. However, since these cured, highly crystalline polyesterslose their rigidity above their crystalline melting points and becomerubber-like in their properties, the present invention is also ofadvantage in pigmenting these substances if they are to be used asrubbers at elevated temperatures. The stabilizing action obtained by thepresent invention is eifective with the cured crystalline polyesterregardless of whether they are used at the low temperatures at whichthey are rigid or at elevated temperatures at which they possess highreversible elasticity.

The present invention is primarily concerned with the stabilizing, orstabilizing and pigmenting, of those elastomers or crystallinesubstances of high tensile strength produced by curing polyesters whichhave achieved an extremely high degree of linear growth, althoughobviously a useful stabilizing and reinforcing action will also beobtained with substances of a lesser degree of linearity.

With the strictly linear polyesters prepared from glycols anddicarboxylic acids containing no non-benzenoid unsaturation, or frommonohydroxy monocarboxylic acids containing no nonbenzenoidunsaturation, the degree of -linear growth is measured directly by themolecular weight of the polyester, sinceitheoretically each molecule ismade up of a single long chain. There is a relatively sharp increase inthe tensile strength of the cured polyesters when the molecular weightsof the'linear polyesters from which they are prepared achieve and exceedvalues in the vicinity of 8,000 to 10,000, as estimated by theStaudinger viscosity method. Linear polyesters of such molecular weightsordinarily possess intrinsic viscosities in chloroform of at least .4.Linear polyesters will also possess such molecular weights if theycontain an average of at least 500 or 600 atoms in their linear chainsor if they contain at least 98 ester groups for each 100 total ester,hydroxyl and carboxyl groups in the polyester (98 per cent oftheoretical complete esterification) Polyesters having essentiallylinear ester chains and formed from reactants, at least one of whichcontains olefinic unsaturation, will possess these high molecularweights associated with high tensile strength if they contain at least98 ester groups per 100 total ester, hydroxyl and carboxyl groups in thepolyester.

If it is desired to produce the high degree of esteriflcation orcondensation indicated above, the reactants from which the polyestersare produced must be subjected to a prolonged heating operation underconditions such as.to remove the reaction by-products continuously andefiectively as described, for instance, in U. S. Patents 2,071,- 250 and2,249,950. The reaction by-products are most effectively removed bybubbling an inert gas v The uncured polyesters may be derived from anyglycols and dicarboxylic acids or any monohydroxy monocarboxylic acidswhich are capable of esterifying to form linear ester chains havinglengths of the order desired Typical saturated glycolsland'dicarboxylicacids are described in U. S. Patent 2,071,250. Unsaturation may beintroduced by substituting an unsaturated dicarboxylic acid such asmuconic, maleic, fumaric, itaconic, mesaconic or dihydromuconic acid fora part of the saturated acid. Similarly, a glycol containing olefinicunsaturation may be substltuted for a portion or all of the saturatedglycol or a hydroxy acid containing olefinic unsaturation may besubstituted for all or part of the saturated hydroxy acid.

In order to produce polyesters of the high degree of linear growthreferred to above which possess good reversible elasticity when cured,it is necessay to limit the amount of unsaturation present in thereaction mixture so that the resulting theoretical polyester which wouldbe produced if no cross-linking occurred would contain less than aboutfive olefinic bonds per 400 atoms in the linear chain and preferablyless than about two such bonds per 400 atoms in the linear chain.

In order to produce cured polyesters which are rubber-like and do notpossess crystalline rigidity at room temperatures, it is necessary toprepare the uncured polyesters from reactants which will yieldpolyesters of a sufiicient degree of non crystallinity to be non-brittleand non-rigid at room temperatures. In producing such noncrystallinepolyesters or polyesters of limited crystallinity, advantage is taken ofthe fact that certain ingredients lead to polyesters which are incapableof crystallization or which have crystalline melting points below roomtemperatures or which crystallize so slowly that for practical purposesthey may be considered permanently non-crystalline.

Polyesters derived by the esterification of polymethylene glycols withpolymethylene dicarboxylic acids or by the esterification ofpolymethylene monohydroxy monocarboxylic acids are, with the exceptionof those produced from trimethylene glycol and glutaric acid, the mosthighly crystalline polyesters which have been produced. As the molecularstructure departs from this straight chain polymethylene arrangement, asfor instance by the introduction of side chain substituents,hetero-atoms or unsaturated carbon-to-carbon bonds, the polyestersbecome less" crystalline.

such as dry, oxygen-free hydrogen through the reaction mixture untilesterification or condensation has proceeded to the desired degree ofcompletion, withor without the application of reduced pressure.

The presence of aromatic rings also in general reduces thecrystallinity.

Therefore, polyesters prepared by reacting glycols with dicarboxylicacids, where either one of the constituents has frequently occurring orlarge side chains, or contains large amounts of non-benzenoidunsaturation or aromatic rings or hetero-atoms in the linear chain, areusually noncrystalline. However, if the other member of the reactionmixture is a polymethylene glycol or a polymethylene dicarboxylic acid,the crystallizing tendencies of the polyester increase as the length ofthe polymethylene chain increases. Thus dihydromuconic acid forms anon-crystalline polyester with ethylene glycol but a crystallinepolyester with decamethylene glycol. Diethylene glycol forms anon-crystalline polyester with succinic acid but a crystalline polyesterwith sebacic acid- Amongv the alkyl substituted polymethylene glycols,the most available is isopropylene glycol or methylethylene glycol. Thisglycol forms non-crystalline polyesters with polymethylene dicarboxylicacids betweenv succinic acid and sebacic acid. Polyisopropylenesuccinate does cessive crystallization.

The most available of the non-crystalline polyester forming reactantscontaining heteroatoms are diethylene glycol and di-isopropylene glycol.Diglycolic acid is also-of some interest as a hetero-atom containingcompound. The most available of the aromatic ring containin reactants isphthalic acid.

Trimethylene glycol and -glutaric acid, both of which contain threemethylene groups between their functional end groups, form polyesters,with the shorter chain polymethylene' glycol and polymethylenedicarboxylic acids, which crystallize exceedingly slowly and aretherefore useful for forming certain of the cured synthetic rubberswhich are not rigid at room temperatures.

Another factor influencing crystallinity, aside from the molecularstructure of the individual constituents, is the degree of order in thepolyester molecules. The most ordered molecules having the mostregularpolar group spacing, all other factors being equivalent, are the mostcrystalline. Thus, the greater the number of glycols and the greater thenumber of dicarboxylic acids or the greater the number of hydroxy acidsused in preparing the polyester, the less will be the tendency tocrystallize. In a polyester prepared from ethylene glycol and togetherwith any other compounding ingredients whichxare to be added. Thecompounded polyester is then heated in a mold to a temperaturesuflicient to effect curing of the polyester.

When the polyesters are more highly crystaland from 5 per cent to 8 percent for succinate polyesters. As the amount of unsaturation increasesthe peak becomes much sharper and the actual amount of peroxide requiredfor curing becomes much less. Thus for a sebacate polyester in which 4mol per cent of the sebacic acid is replaced by maleic acid the optimumamount of peroxide is from .5 per cent to 1 per cent. For acorresponding succinate maleate polyester the optimum amount is from 1per cent to 2 per cen Benzoyl peroxide isthe most effective curing agentyet found, but other acyl peroxides, such as lauryl peroxide, aresuitable although less effective. Certain other organic peroxides suchas certain-of the ether peroxides, ketone peroxides, olefln peroxides,terpene peroxides (particularly ascaridol), peracids and hydrocarboncium carbonates, and particularly the ultra-fine line milling may becarried on with hot rolls maintained above the crystalline melting pointbut below the curing temperature. The ingredients may also be mixed insolution, regardless of whether the polyester is normally crystalline ornon-crystalline.

The amount of curing agent required to give maximum tensile strengthvaries with the composition of the polyester, being afiected both byperoxides are sufliciently eii'ecti've to render them usable for curingthe polyesters. particularly those containing unsaturation.

As indicated above, the stability of those polyesters cured withbenzoylperoxide or other acyl peroxides, in the absence of any stabilizingagent, is dependent upon the amount of peroxide used for curing, thosepolyesters cured with the most peroxide being the least stable. Thegreatest effect in improving stability by the incorporation of thebuflering agents of the present invention is therefore, observed withthose polyesters cured with the greatest amount of acyl peroxide.

The amount of buffering agent to be added is governed primarily by thephysical effect of the finely divided solid on the properties of thepolyester. Ordinarily more than about 1 per cent of the buffering agentby weight of the polyester will be used. With the relatively finecalcalcium carbonate referred to above, amounts up to about 100 per centby weight or more may be used. I

When it is desired to take advantageof the outstanding reinforcingaction of the ultra-fine calcium carbonates in the cured polyesterswhich are rubber-like under the conditions of use, this pigment willordinarily be added in substantial quantities. The reinforcing effect ofthe pigment is dependent upon the amount of pigment used. 'Iihus apolyester made up of 50 mol per cent ethylene glycol, 5 mol per centisopropylene glycol, 97 mol per cent succinic acid-and 3 mol per centmaleic acid, when cured with 1.5 per cent-benzoyl peroxide in theabsence of any pigment, had a tensile strength not substantially greaterthan 100 pounds per square inch. This same rubber reached a maximumtensile strength in the vicinity of 2500 pounds per square inch creasedfrom these val-lies the tensile" strength fell on sharply, having avalue of about 1700 pounds per square inch with .25 per cent p18- mentrAbove the optimum pigment content the tensile strength fell ofl slowly.For other polyester rubbeis the optimum percentages for this pigment aresubstantially the same.

The following specific examples will illustrate the stabilizing actionat the buffering agents and the remarkable reinforcing action of theultra-fine calcium carbonate:

Example I.-A mixture of dicarboxylic acids containing 98 mol per centsuccinic acid and 3 mol per cent maleic acid and a 25 mol per centisopropylene glycol and 50 mol per cent ethylene glycol, together with asmall amount of zinc chloride as a catalyst, were placed in a closedglass reaction vessel maintained at 200 C., and a slow stream of dry,oxygen-free hydrogen was bubbled continuously through the reactionmixture. A lpacked reflux column maintained at 110 C. was attached tothe reaction vessel. After about five hours no more water was evolved,indicating that substantially complete esterification had occurred. Thereflux column was then removed and the pressure in the system wasreduced to about 6 millimeters of mercury, the temperature beingmaintained at about 200 C. and the bubbling of hydrogen being continued.Glycol distilled over rapidly and after about minutes an increase in theviscosity of the produce was apparent. At the end of about eight hoursthe product was removed and found to be an ex-ceedingly viscous,transparent liquid. Two portions of this viscous liquid were milledseparately on cold rolls together with about 9 per cent of finelydivided benzoyl peroxide. Sixty per cent by weight of Gastex carbonblack was then added to each batch of gum on the rolls and the millingexcess-of a mixture containing mol per cent maleate residues, saidpolyester prior to curing having a degree of linear growth indicated by8 ratio of ester groups to total ester, hydroxyl and residues, themaleate residue constituting about was continued until the pigment waswell dlspersed. In addition, 10 per cent by weight of Atomi-te (finelydivided calcium carbon-ate) was milled into one batch of gum. Each batchwas 0 both batches were maintained immersed in separa-te test tubes inwater maintained at C. The samples were tested periodically to determinetheir rate of deterioration by measuring their elongation under a 500gram load. After two days, the sample containing no calcium carbonatehad deteriorated to the point where the two-inch sample stretched tofour inches under the 500 gram load. The sample containing calciumcarbonate required seven days of immersion to show the same degree ofdegradation.

Example II.-An uncured polyester was prepared under the conditionsdescribed in Example I using a reaction mixture made up of 50 mol percent sebacic acid, 40 mol per cent isopropylene glycol and 10 mol percent ethylene glycol. The resulting polyester was milled on cold rollsfirst with 3 per cent benzoyl peroxide and then with '75 per cent byweight of Kalvan. The compounded polyester was cured for ten minutes at125 C. The resulting product had a tensile strength of 3200 pounds persquare inch at an elongation of 650 per cent.

Although the invention been described in terms of its specificembodiments, certainmodifications and equivalents will be apparent tothose skilled in the art and are intended to be included within thescope of [the invention which is to be limited only by the reasonablescope of the appended claims. I

What is claimed is:

1. Benzoyl peroxide cured polyisopropylene ethylene sebacate male-ate.having between 50 per cent and 75 per cent by weight of finely divided,precipitated calcium carbonate having an average particle size of theorder of .03 micron or less dispersed therethrough, the isopropyleneresidue in said polyester constituting at least 80 mol per cent of thecombined isopropylene and ethylene residues, the maleate residueconstituting about 3 mol per cent of the combined sebacate and 3mol percent of the combined succinate and maleate residues, said polyestersprior to curing having a degree of linear growth indicated by a ratio ofester groups to total ester, hydroxyl and carboxyl groups of at least 98per cent.

3. Benzoyl peroxide cured polyisopropylene ethylene sabacate maleateintimately mixed with finely divided calcium carbonate, said polyesterprior to curing having a degree of linear growth indicated by a ratio ofester groups to total ester, hydroxyl and carboxyl groups of at least 98per cent, said polyesters containing less than two olefinic bonds per400 atoms in the linear ester chains, calculated by assuming nocross-linking between molecules at .the unsaturated bonds.

4. Benzoyl peroxide cured polyisopropylene ethylene succlnate maleateintimately mixed with finely divided calcium carbonate, said polyesterprior to curing having a degree of linear growth indicated by a ratio ofester groups to total ester, hydroxyl and carboxyl groups of at least 98per cent, said polyesters containing less than two olefinic bonds per400 atoms in the linear ester chains, calculated by assuming nocross-linking between molecules at the unsaturated bonds.

5. Benzoyl. peroxide cured polyisopropylene ethylene sebacate intimatelymixed with finely divided calcium carbonate, said polyester prior tocuring having a degree of linear growth indicated by a ratio of estergroups to total ester, hydroxyl and carboxyl groups of at least 98 percent, said polyesters containing less than two olefinic bonds per 400atoms in the linear ester chains, calculated by assuming nocross-linking between molecules at the unsaturated bonds.

6. A cured elastomer having intimately mixed therewith a finelydividedreinforcing and stabilizing pigment in an amount between about 50 percent and about 75 per cent by weight of the elastomer, said pigmentbeing precipitated calcium carbonate having an average particle size ofthe order of .3 micron or less, said elastomer being a, benzoyl peroxidecured dlhydroxyalkane-dicarboxyalkane-dicarboxyolefin polyester, whereinthe dicarboxyolefin and dicarboxyalkane are so proportioned that thepolyester contains less than five olefinic bonds per 400 atoms in thelinear ester chains, calculated by assuming no cross-linking betweenmolecules at the unsatu- 9 less, said elastomer bein an organic peroxidecured dihydroxy aliphatic hydrocarbon-dicarboxy hydrocarbon polyester,said polyester prior to curing containing, as the sole non-benzenoidunsaturation less than five olefinic bonds per 400' atoms in the linearester chains, calculated by assuming no cross-linking between moleculesat the unsaturated bonds, said polyester possessing a suflicient degreeof non-crystallinity to permit it to be milled on cold rolls, saidpolyester having a degree of linear growth indicated by a ratio of estergroups to total ester, hydroxyl and carboxyl groups of at least 98 percent.

8. A polymer composition comprising finely divided calcium carbonateintimately mixed with a benzoyl peroxide cured polyester made upessentially of divalent hydrocarbon radicals joined by carboxylic esterlinkages into essentially linear ester chains, said chains possessing adegree of linear growth as indicated by a ratio of ester groups to totalester, hydroxyl and carboxyl groups I benzenoid unsaturation, less thanfive olefinic Number Name Date 2,195,362 Ellis Mar. 26, 1940 2,255,313Ellis Sept. 9, 1941 OTHER REFERENCES The Valderbilt Rubber Handbook,1942, pages of which are essentially linear, said polyesterv possessinga degree of linear growth indicatedby a ratio of ester groups to totalester, hydroxyl and carboxyl groups 01' at least 98 per cent, saidpolyester containing, as the sole non-benzenoid unsaturation, less thanfive oleflnic bonds per 400 atoms in the linear ester chains.

10. A polymer composition comprising a finely divided alkaline carbonateintimately mixed with an acyl peroxide cured polyester the esterlinkages of which are carboxylic ester groups, the ester chains of whichare essentially linear, said polyester possessing a degree of lineargrowth indicated by a ratio of ester groups to total ester, hydroxyl andcarboxyl groups of at least 98 per cent, said polyester containing, asthe sole nonbonds per 400 atoms in the linear ester chains.

BURNARD S. BIGGS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

